1. Field of the Invention
The present invention relates generally to a backlight unit installed in a Liquid Crystal Display (LCD) and, more particularly, to a backlight unit that is capable of effectively dissipating the large quantity of heat that is generated by Light-Emitting Diodes (LEDs), which are light sources.
2. Description of the Related Art
A backlight unit is a device that is installed in an LCD using the principle in which liquid crystals change their molecular arrangement according to applied voltage, and provides light and illuminates a screen from behind. Although such backlight units were mainly formed of cold-cathode tubes, LEDs have attracted attention as backlight units due to the advantages with respect to life span, brightness, color reproducibility, etc.
When LEDs are used as light sources, LEDs require substrates, unlike cold-cathode tubes. Since LEDs emit large quantities of heat while radiating light, metal core substrates (metal core printed circuit boards) having an excellent heat dissipation characteristic have been used. Although metal core substrates have an excellent heat dissipation characteristic, they are very expensive. Accordingly, the high cost of the metal core substrates is one of the principal factors that impede the cost competitiveness of the backlight units formed of the metal core substrates. As a result, there is a trend toward the use of relatively inexpensive epoxy resin insulating substrates. An example of a conventional backlight unit in which LEDs are mounted on such an insulating substrate is illustrated in FIG. 4.
As illustrated in FIG. 4, a backlight unit 200 includes an insulating substrate 210, a plurality of LED packages 230 and a chassis 250.
Circuit patterns 211 and 212 are formed on the insulating substrate 210 by coating an epoxy resin FR4-core with a copper foil and etching the copper foil.
Each of the LED packages 230 is mounted such that an LED chip 231 is directly connected to one LED electrode 232 and is wire-bonded to the other LED electrode 233.
The LED chip 231 and the LED electrodes 232 and 233 are placed within a plastic mold casing 234, and the casing 234 is covered with an epoxy resin lens 235.
The LED package 230 is mounted on the insulating substrate 210, and is electrically connected to positive and negative electrodes, that is, the circuit patterns 211 and 212.
The chassis 250 is made of material having excellent thermal conductivity, such as metal, and is placed below the insulating substrate 210 with a heat pad 270 placed therebetween so as to provide electrical insulation and decrease contact thermal resistance.
In the conventional backlight unit 200, a thermal resistance obtained using the equation R=L/KA is about 63.5 K/W, which is very high, when the thickness L of a substrate is 0.8 mm, the thermal conductivity K of the substrate is 0.35, and the area A of the substrate is 36 mm2.
As a result, the conventional backlight unit having the above-described construction exhibits the very poor thermal conductivity of the insulating substrate, therefore it is difficult to effectively eliminate heat generated by the LED chips, with the result that the temperature of the LED chips continuously increases. Accordingly, the amount of light emitted by the LED chips decreases, variation in wavelength occurs, and the reliability of the LED chips decreases, thus resulting in a reduced lifespan.